The present invention relates to a dielectric filter and in particular to a small dielectric filter suitable for use in a high frequency band equal to or higher than 3 GHz.
With the spread of mobile communication device, a frequency band higher than that in current operation is considered to be made use of. In the conventional mobile communication, the frequency band up to about 2 GHz is used, and a combination of dielectric coaxial resonators has been mainly employed as a filter used in the mobile station.
When the dielectric coaxial resonator is used, however, in the frequency band equal to or higher than 3 GHz, an axial dimension thereof has to be made shorter due to the frequency, which makes it extremely thinner and also makes it difficult to form an input and output coupling. In addition, to secure high Q, an outer diameter of the dielectric shall be made larger. For example, in order to secure a Q required at a frequency of 5 GHz, 10-odd mm of outer diameter is necessary. This goes against a requirement for making an electronic unit smaller and is not practical. Instead of coaxial TEM mode resonator, TE mode resonator may be considered to be used, which results in larger size of structure and requires a complex structure of input and output coupling.
The object of the present invention is to provide a dielectric filter, which provides sufficient filtering characteristic at high frequency band, for example, within the range of 3 GHz to 30 GHz, and meets the requirement for high Q, downsizing and thinner thickness.
The present invention solves the problems in the prior art described above by employing an entirely new structure quite different from conventional ones.
That is, the present invention provides a dielectric filter composed of three or more rectangular parallelepiped dielectrics connected in line, the dielectric filter characterized in that:
an input/output electrode made up of island type of conductive film is formed on one surface of the dielectric located on each end portion;
in each of the dielectrics located on each end respectively, an earth electrode is formed on almost all of the remaining area of the one surface so as to be isolated from the input/output electrode and is also formed on all of the other surfaces with an exception of connecting surfaces;
in an intermediate dielectric, an earth electrode is formed on all surfaces other than the connecting surface; and
a conductive film connected to the earth electrode is formed on a part of at least one of the connecting surfaces of the dielectrics to be connected.
Three or more elements of resonators may be integrally formed on a dielectric block, and, in that case, a through-hole is formed between the resonators.